Untransformed display lists in a tile based rendering system

ABSTRACT

A three-dimensional computer graphics rendering system allows a tile-based rendering system to operate with a reduced amount of storage required for tiled screen space geometry by using an untransformed display list to represent the screen&#39;s geometry.

This invention relates to a three-dimensional computer graphicsrendering system and in particular to methods and apparatus associatedwith rendering three-dimensional graphic images utilising anuntransformed display list within a tile based rendering system.

BACKGROUND TO THE INVENTION

Tile based rendering systems are well known, these subdivide an imageinto a plurality of rectangular blocks or tiles in order to increaseefficiency of the rasterisation process.

FIG. 1 illustrates a traditional tile based rendering system. Tile basedrendering systems operate in two phases, a geometry processing phase anda rasterisation phase. During the geometry processing phase aprimitive/command fetch unit 100 retrieves command and primitive datafrom memory and passes this to a geometry fetch unit 105 which fetchesthe geometry data 110 from memory and passes It to a transform unit 115.This transforms the primitive and command data into screen space andapplies any lighting/attribute processing as required using well-knownmethods. The resulting data is passed to a culling unit 120 which cullsany geometry that isn't visible using well known methods. The cullingunit writes any remaining geometry data to the transformed parameterbuffer 135 and also passes the position data of the remaining geometryto the tiling unit 125 which generates a set of screen space objectslists for each tile which are written to the tiled geometry lists 130.Each object list contains references to the transformed primitives thatexist wholly or partially in that tile. The lists exist for every fileon the screen, although some object lists may have no data in them. Thisprocess continues until all the geometry within the scene has beenprocessed.

During the rasterisation phase the object lists are fetched by a tiledparameter fetch unit 140 which first fetches the object references andthen the object data referenced and supplies them to a hidden surfaceremoval unit (HSR) 145 which removes surfaces which will not contributeto the final scene (usually because they are obscured by anothersurface). The HSR unit processes each primitive in the tile and passesonly data for visible primitives/pixels to a texturing and shading unit(TSU) 150. The TSU takes the data from the HSR unit and uses it to fetchtextures and apply shading to each pixel within a visible object usingwell-known techniques. The TSU then supplies the textured and shadeddata to an alpha test/fogging/alpha blending unit 155. This Is able toapply degrees of transparency/opacity to the surfaces again usingwell-known techniques. Alpha blending is performed using an on chip tilebuffer 160 thereby eliminating the requirement to access external memoryfor this operation. It should be noted that the TSU and alphatest/fogging/alpha blend units may be fully programmable in nature.

Once each tile has been completed, a pixel processing unit 165 performsany necessary backend processing such as packing and anti-aliasfiltering before writing the resulting data to a rendered scene buffer170, ready for display.

Typically modern computer graphics applications utilise a significantamount of geometry that remains static throughout a scene or acrossmultiple scenes, this geometry data is stored in what is commonly knownas static vertex buffers that typically reside in memory that is localto the graphics processing unit. Current tile based systems transformthis data into screen space and store the resulting geometry within aparameter buffer/tiled screen spaced geometry list that can consume aconsiderable amount of additional storage and memory bandwidth.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a method andapparatus that allow a tile based rendering system to operate with areduced amount of storage required for tiled screen space geometry. Thisis accomplished by the use of an untransformed display list to representthe scene's geometry. This removes the need for the transformedparameter buffer 135 In FIG. 1 by utilising the fact that the incomingscene geometry is static and so it can be referenced in both thegeometry processing and rasterisation phases.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described in detailby way of example with reference to the accompanying drawings in which:

FIG. 1 illustrates a traditional tile based rendering system;

FIG. 2 illustrates a tile based rendering system using an untransformeddisplay list;

FIG. 3 illustrates deferred lighting/attribute processing;

FIG. 4 illustrates the addition of a transformed data cache to thesystem; and

FIG. 5 illustrates a hybrid transformed/untransformed display list basedtile based rendering system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 illustrates a tile based rendering system that has been modifiedto support an untransformed display list. During the geometry processingphase a primitive/command fetch unit 200 retrieves command and primitivedata from memory and passes this to a position data fetch unit 205 whichfetches a position part of static geometry data from memory 210 andpasses it to transform 1 unit 215. This transforms the primitive intoscreen space only i.e. it does not apply any lighting/attributeprocessing as would occur in the system of FIG. 1. The resulting screenspace position data is passed to a culling unit 220 which culls anygeometry in the same manner as the system of FIG. 1. Unlike the systemof FIG. 1 the culling unit does not write the remaining geometry data toa transformed parameter buffer, instead It only passes the position dataof the remaining geometry to a tiling unit 225.

In the system of FIG. 1, the tiling unit generates references totransformed geometry that has been stored in the transformed parameterbuffer, in the new system the tiling unit generates references to theuntransformed static geometry data which are written to the tiledgeometry lists 230 as before. These references are in the form ofpointers to the geometry data in the memory 210. This process continuesuntil all the geometry within the scene has been processed.

During the rasterisation phase object lists for each tile are fetched bya tiled parameter fetch unit 240 which supplies the static geometryreferences (pointers) from the total geometry lists to untransformedgeometry fetch unit 245 which fetches the untransformed static geometrydata from memory 210 and passes it to the transform 2 unit 250. Thetransform 2 unit retransforms the retrieved data to screen space andapplies any required lighting/attribute processing etc to the geometry.The transformed geometry is then passed to hidden surface removal unit(HSR) 255 which removes surfaces which will not contribute to the finalscene as In the system of FIG. 1. The remaining stages 260 through to280 all operate in the same manner as stages 150 through 170 (in FIG. 1)as described above. [John: should FIG. 2 also include a source ofdynamic geometry?]

In a further optimisation it is possible to defer any lighting orattribute processing that is required after hidden surface removal hasbeen performed. This means that this processing is only applied to thatgeometry which is visible within the final scene giving significantimprovements in both throughput and power consumption. FIG. 3illustrates a modification to the system that implements deferredlighting/attribute processing. Units 300 and 305 operate as describedfor units 240 and 245 of FIG. 2, unlike unit 250 in FIG. 2 the transform2 unit 310 only transforms the position data before passing it onto thehidden surface removal unit 315. The visible primitives emitted by thehidden surface removal unit are then passed to transform 3 unit 320where any lighting/attribute processing is performed. The operation ofunits 325 to 350 is the same as units 145 to 170 in FIG. 1.

It should be noted that each of the three transformation units mentionedabove could all be implemented in a single “universal” unit similar tothat described in our British Patent Application GB-A-2430513. Althoughthe above approaches eliminate the need for a transformed parameterbuffer they have the disadvantage of requiring the position data to betransformed in both phases and for the transformation to be repeated forevery tile that any piece of geometry overlaps. FIG. 4 Illustrates amodification to the rasterisation phase of the untransformed displaylist system in which a cache is added in order to minimise the number oftimes the data is retransformed in the rasterisation phase. It should benoted that although FIG. 4 shows a modification with respect to a nondeferred lighting/attribute processing system It is equally applicableto either. As in FIG. 2 the tiled parameter fetch unit 400 fetches thetiled object list references generated in the geometry processing phasefrom memory. The references are passed to a cache control unit 405 whichchecks to see if there is an entry In the transformed data cache memory410 that corresponds to the object reference, if there is the cachecontrol unit reads the data from the cache and passes it to the hiddensurface removal unit 425. If there is no corresponding entry in thecache the cache control unit issues the reference to the untransformedgeometry fetch unit 415 which fetch the data from memory and passes itto the transform 2 unit 420. The transform 2 unit transforms and appliesany lighting/attribute process required to the geometry data and thenpasses it back to the cache control unit. The cache control unit thenadds it to the transformed data cache memory for future reference beforepassing it to the hidden surface removal unit. The operation of units425 to 450 is the same as units 145 to 170 in FIG. 1.

In order to eliminate the additional geometry processing pass used inthe above approach the result of the position transform can be stored ina parameter buffer for use in the second pass. Although this results inthe need for transformed parameter storage it may be consider a usefultrade off compared against transforming the position data multipletimes. It should also be noted that there are cases were an applicationwill update the vertex data during a scene, this type of vertex data isoften referred to as dynamic, in these circumstances the data must betransformed and copied to a parameter buffer as per a conventional tilebased rendering device.

FIG. 5 illustrates a hybrid system that allows the use of bothuntransformed and transformed display lists. During the geometryprocessing phase a primitive/command fetch unit 500 retrieves commandand primitive data from memory and passes this to the geometry fetchunit 505 which fetches both the dynamic geometry data 507 and staticgeometry data 510 from memory and passes it to the transform 1 unit 515.

For dynamic geometry the transform 1 unit transforms the position andapplies any required lighting/attribute processing as per a traditionaltile based rendering system, for static geometry only the position istransformed as previously described. The resulting data is passed to aculling unit 520 which culls any geometry that isn't visible using wellknown methods. The culling unit writes any remaining dynamic geometryand static position data to the transformed parameter buffer 535 andalso passes the position data of the remaining geometry to the tilingunit 525 which generates a set of screen objects lists for each tilewhich are written to the tiled geometry lists 530. It should be notedthat the tiled geometry lists indicate which geometry is dynamic andwhich is static. As in FIG. 2 the tiled parameter fetch unit 540 fetchesthe tiled object list references generated in the geometry processingphase from memory. The references are passed to the cache control unit545 which checks to see if there is an entry in the transformed datacache memory 550 that corresponds to the object reference, if there isthe cache control unit reads the data from the cache and passes it tothe hidden surface removal unit 565. If there is no corresponding entryin the cache the cache control unit issues the reference to either thetransformed parameter fetch unit 547 or the untransformed geometry fetchunit 555 based on the type indicated in the tiled reference lists.Transformed geometry is fetched by the transformed parameter fetch unitand passed back to the cache control unit and untransformed geometry isfetched by the untransformed geometry fetch unit and processed bytransform unit 2 560 before being passed back to the cache control unitBoth geometry types are then written to the cache by the control unitbefore being passed to the hidden surface removal unit All subsequentunits 665 through to 590 operate as previously described for units 145through 170 in FIG. 1.

1. A method for reducing parameter memory usage in a tile based rendering system comprising the steps of: retrieving position data from stored static geometry in a memory; transforming the retrieved position data to screen space; using the screen space position data to compile a list of corresponding pointers to static geometry data in memory, from which the position data was retrieved, for each rectangular area; for each rectangular area retrieving pointer data from each list; retrieving static geometry data from the memory corresponding to the retrieved pointer data for each list; transforming the thus retrieved position geometry data to screen space; and applying any required attribute processing prior to applying hidden surface removal; and rendering the transformed data to a buffer for display.
 2. A method according to claim 1 including the step of lighting or attribute processing after the step of hidden surface removal.
 3. A method according to claim 1 in which the step of transforming geometry data to screen space image data includes the step of storing transformed data in a cache, and the step of retrieving geometry data includes the step of checking whether data to be retrieved is present in the cache.
 4. A method according to claim 1 in which the transformed position data is written to a parameter buffer such that it need not be transformed a second time during the processing of a rectangular area.
 5. A method according to claim 1 further comprising the steps of: retrieving dynamic geometry data from a second memory; transforming the retrieved data to screen data position and image data; wherein the compiling steps comprises a list of pointers to screen space data for the dynamic geometry data and untransformed for static geometry data; and including the step of storing the transformed dynamic geometry data; and wherein the step of retrieving untransformed static geometry data further comprises retrieving transformed dynamic geometry data.
 6. A tile based rendering system with reduced parameter memory comprising; means for retrieving position data from stored static geometry data in a memory; means for transforming retrieved position data to screen space; means for using the screen space position data to compile a list of corresponding pointers to static geometry data in the memory from which position data is retrieved for each rectangular area; means for retrieving for each rectangular area pointer data from each respective list; means for retrieving static geometry data from the memory corresponding to the retrieved pointers for each list; means for transforming the thus retrieved position geometry data to screen space; and, means for rendering the transformed data to a buffer for display.
 7. A rendering system according to claim 6 including means for applying lighting or attribute processing after the means for hidden surface removal.
 8. A rendering system according to claim 6 in which the means for transforming geometry data to screen space includes means for storing transformed data in a cache, and the means for retrieving geometry data includes means for checking where the data to be retrieved is present in the cache.
 9. A rendering system according to claim 6 including means to write transformed position data to a parameter buffer such that subsequent transformation is not required during the processing of a rectangular area.
 10. A rendering system according to claim 6 further comprising; means for retrieving dynamic geometry data from a second memory; means for transforming the retrieved dynamic geometry data to screen space position and image data; wherein the means for compiling a list of pointers to screen space data includes means to comprise a list of pointers to screen space data for the dynamic geometry data and for static geometry data; means for storing transformed dynamic geometry data; and wherein the means for retrieving untransformed static geometry data further includes means for retrieving transformed dynamic geometry data.
 11. A method for reducing parameter memory usage in a tile based rendering system substantially as herein described.
 12. A rendering system with reduced parameter memory substantially as herein described with reference to the accompanying drawings. 